The invention relates to a plasma picture screen provided with a front plate comprising a glass plate on which a dielectric layer and a protective layer are provided, with a carrier plate provided with a phosphor layer, with a ribbed structure which subdivides the space between the front plate and the carrier plate into plasma cells, which are filled with a gas, and with one or several electrode arrays on the front plate and the carrier plate for generating corona discharges in the plasma cells.
Plasma picture screens render possible color pictures with high resolution, large screen diameter, and have a compact construction. A plasma picture screen comprises a hermetically closed glass cell which is filled with a gas, with electrodes in a grid arrangement. The application of a voltage causes a gas discharge which generates mainly light in the vacuum ultraviolet range. This VUV light is converted into visible light by phosphors and is emitted through the front plate of the glass cell to the viewer.
Plasma picture screens are subdivided into two classes: DC plasma picture screens and AC plasma picture screens. The electrodes in DC plasma picture screens are in direct contact with the plasma. In AC plasma picture screens, the electrodes are separated from the plasma by a dielectric layer.
In principle, two types of AC plasma picture screens can be distinguished: with a matrix arrangement and with a coplanar arrangement of the electrode arrays. In the matrix arrangement, the gas discharge is ignited and maintained at the intersection of two electrodes on the front and on the carrier plate. In the coplanar arrangement, the gas discharge is maintained between the electrodes on the front plate and is ignited at the point of intersection with an electrode, a so-called address electrode, on the carrier plate. The address electrode in this case lies below the phosphor layer.
The dielectric layer in a typical AC plasma picture screen is covered by an additional MgO layer. MgO has a high ion-induced secondary electron emission coefficient and thus reduces the ignition voltage of the gas. In addition, MgO is resistant to sputtering by positively charged ions of the plasma. A disadvantage is, however, that MgO is readily contaminated by foreign substances during the manufacturing process, which substances are very difficult to remove afterwards.
JP 11054048 A of Patent Abstracts of Japan discloses an AC plasma picture screen which has a protective layer of diamond-resembling carbon (amorphous diamond) on the dielectric layer instead of a protective layer of MgO. The protective layer has an amorphous structure and is deposited in a CVD (Chemical Vapor Deposition) process.
A disadvantage of the use of diamond-like carbon in the protective layer is that diamond-like carbon may change its structure and may partly also evolve hydrogen under the rigid conditions, for example high temperatures, prevailing in the manufacture of plasma picture screens. A disadvantage of the structure change is that a layer with a graphite content is formed thereby, which shows a brownish discoloration. This reduces the luminance of the plasma picture screen. Evolved hydrogen may change the gas phase inside the plasma picture screen, so that, for example, the ignition voltage may change in an uncontrolled manner.
The invention has for its object to provide an improved plasma picture screen.
This object is achieved by means of a plasma picture screen provided with a front plate comprising a glass plate on which a dielectric layer and a protective layer are provided, with a carrier plate provided with a phosphor layer, with a ribbed structure which subdivides the space between the front plate and the carrier plate into plasma cells, which are filled with a gas, and with one or several electrode arrays on the front plate and the carrier plate for generating corona discharges in the plasma cells, wherein the protective layer comprises a material chosen from the group of crystalline diamond, AlN, AlGaN, BN, and tetrahedral amorphous carbon.
These materials have a high chemical resistance, for example to high temperatures in the manufacture of the plasma picture screen, and compared with MgO they are not hygroscopic. They also show a higher physical resistance than diamond-resembling carbon and are, for example, sputter resistant to high-energy plasma ingredients. In addition, a protective layer made of one of these materials contains no appreciable quantities of hydrogen, and a change in the gas phase in the discharge cells through the generation of hydrogen is prevented.
It is furthermore preferred that the gas comprises xenon in a relative quantity of more than 7% by volume.
A protective layer of crystalline diamond, AlN, AlGaN, BN, or tetrahedral amorphous carbon renders it possible to increase the xenon quantity in the gas without a major rise in the ignition temperature. The increase in the proportional quantity of UV light generating xenon in the gas renders the UV radiation generation, and thus the excitation of the phosphors more efficient.